Wih 2 strokes I tend to use 5 or 6 pitch props because I have a wide range of rpm's I can run them at.
With 4 strokes I pretty much limit the motors at 10,000 rpm so I tend to run 8 and 9 pitch props to grab more air at the lower rpm's. Just my $.02
There is no reason you can't run a 13x6. To me a Venus looks like a pattern plane and pattern planes usually fly at higher speeds.
It comes down to a matter of personal preference. Run what works best for you and the plane. Enjoy your Venus. Tom